one thing they are still not exploring is the use of soil or bedrock for thermal storage even though there are at least two projects doing that. I guess I will have to make my own project and make some noise about it.

It has been a while so my memory may not be perfect, but it seems there was a project in Spain, maybe, that was using underground storage via melted salt compounds. It may have only been in the planning stages, but I remember the discussion of how the energy would cycle vertically.

_________________With friends like Guido, you will not have enemies for long.

“Intellect is invisible to the man who has none” Arthur Schopenhauer

"The difference between genius and stupidity is that genius has its limits."Albert Einstein

I think it is a cultural thing but electricity use in Spain varies dramatically on a daily cycle. A general lack of natural resources and windy areas leads Spain to concentrate on solar. The peak use is after dark which means solar on houses does little to help. All of Spain's current and recently commissioned projects appear to be large-scale solar with a few hours of storage. All of the methods I have heard of are represented in one of the Spain projects. Yes, molten salts are one of the most common fluids in Spain's solar systems (most others use oil). The first to use storage had large blocks of concrete that kept the turbines running when clouds passed over. Longer term used two tanks, one with the hottest liquid (300-400 C) and the other with the liquid that the heat had been removed to create steam to drive the turbines (200-300 C). To reduce cost, a newer project used one tank but the tank was stratified (hottest on top, coldest on bottom). Another idea used first in Spain is to store super-heated and compressed water. The pressure kept it liquid but as soon as the pressure was lowered, it "flashed" to steam and was added to the regular steam (or replaced it) for very quick responding replacement of lost solar input (and would store about 2 hours of after-dark capacity). This tank was cost effective at a smaller size but high pressure tanks and valves get expensive when scaled up. I heard that Spain is planning to make electricity export a significant part of their GDP (Gross Domestic Product) so are building lots of new solar plants and I expect more designs will show up.